I asked this question on Aug 10 and got a reply from Mark Proctor saying the only way to currently do this is as follows:
"you would have to call drools.getWorkingMemory().startProcess(id) from inside of a consequence."
This means that you would have to create a rule that is always guaranteed to fire first e.g. with a "when" condition of true and a salience that is higher than all the other rules. The "then" part of the rule will be a call to
drools.getWorkingMemory().startProcess(id).
Personally I don't like this, as there should really be an API option to set a ruleflow to a stateless session. Do a search for statelesssession + ruleflow in the rules-users archives for a the original thread (started on Aug 10).
Regards
Shahad